The process of producing a polymer based on conjugated alkadiene is well known in the art. Such processes are typically conducted in the presence of a hydrocarbon reaction diluent and an organolithium initiator. In the polymerization of conjugated alkadiene, two types of polymerization take place. In what is conventionally termed 1,2 polymerization (or alternatively 3,4 polymerization), the polymerization involves only a single carbon-carbon double bond. The resulting polymer chain includes the carbon atoms of that bond and contains a pendant vinyl group. The extent to which such polymerization takes place is measured by the proportion of such vinyl groups in the polymer and is conventionally referred to as vinyl content. In what is termed 1,4 polymerization, all four carbon atoms of the conjugated system are incorporated into the polymer chain which then includes ethylenic unsaturation.
The vinyl content of polymers formed from conjugated alkadiene is on the order of 10% unless steps are taken to increase the proportion. It is often desirable from consideration of polymer properties to have higher vinyl contents, even as high as contents on the order of 60%. Increased vinyl content is achieved by well known methods through the use of a structure modifier to alter the proportions of 1,2 and 1,4 polymerization of an alkadiene.
One group of widely used structure modifiers is the group of acyclic polyethers such as diethylene glycol dialkyl ether or alkyl orthoesters. However, these modifiers are not entirely satisfactory. The diethylene glycol dialkyl ethers, e.g., diethylene glycol dimethyl ether, have insufficient stability during polymerization and provide protic or other polar derivatives which tend to prematurely terminate polymerization. The ortho esters, such as those disclosed by Onishi et al, U.S. Pat. 3,580,895, appear to decompose during the steam coagulation by which the polymer is conventionally recovered. The decomposition products are observed in the recovered hydrocarbon reaction diluent which must be purified for most efficient use if the diluent is recycled.
A second group of structure modifiers comprises cyclic ethers including cyclic polyethers. Acyclic monoether, i.e., tetrahydrofuran, is known to be relatively ineffective unless used in high concentration. Some cyclic polyethers containing a--linkage, e.g., dioxolane and paraformaldehyde, are described by House, U.S. Pat. No. 3,288,872. Such cyclic polyethers, however, tend to inhibit organoalkali metal initiated polymerization.
It would be of advantage to provide a process for conjugated alkadiene-based polymerization using an improved structure modifier.